The epidermal growth factor receptor (EGFR) is overexpressed during bladder cancer progression, and clinically relevant EGFR antagonists inhibit the growth of tumor xenografts in vivo. However, EGFR inhibitors not displayed consistent activity in clinical trials performed in other disease sites, which has dampened clinical enthusiasm for aggressively developing them further. Studies performed within the context of non-small cell lung cancer demonstrated that clinical responses were linked to activating mutations within the EGFR tyrosine kinase domain, suggesting that a better understanding of the biological effects of EGFR inhibitors on tumor cells will help to identify tumors that will respond (biologically or clinically) to therapy. In preliminary studies conducted during the first cycle of SPORE funding, we found that EGFR inhibitors (gefitinib or cetuximab) blocked cell cycle progression in 8/20 human bladder cancer cell lines, and we have identified molecular mechanisms that appear to mediate these effects. Although none of the cell lines nor any of 75 primary tumors contained activating EGFR kinase domain mutations, preliminary data suggests that over 50% of primary tumors express a truncated form of the EGFR (EGFRvlll) that mediates ligand-independent signaling and promotes tumor growth in other model systems. Therefore, the overall goal of this project is to define the biological properties associated with EGFR-dependent growth in bladder cancer cells and to develop pharmacodynamic markers that can be used to monitor them in tissue biopsies harvested during therapy. To this end, we propose 4 Specific Aims. (1) Define the molecular mechanisms underlying the antiproliferative effects of EGFR inhibitors in urothelial carcinoma cells. (2) Determine the functional significance of EGFR-vlll expression in urothelial carcinoma. (3) Define the role of the host response in the angiogenesis inhibition observed in response to EGFR-directed therapy. (4) Evaluate the activity of combined therapy with EGFR inhibitors and TRAIL. An important component of this project is a clinical trial designed to test the effects of cetuximab on pharmacodynamic markers in primary tumors. Thus, we are in a unique position to directly test the validity of the hypotheses we have generated in our preclinical studies and to exploit them in the design of more effective, EGR-based therapeutic strategies.